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NEENA B. SCHWARTZ and WILLIAM L. TALLEY

In female rats, uterine weight increases early in pro-oestrus (Astwood, 1939; Schwartz, 1964), behavioural oestrus occurs during the early evening of prooestrus (Boling, Blandau, Soderwall & Young, 1941) and pituitary lh content drops from a maximum at pro-oestrus to a minimum on the morning of oestrus (Schwartz & Bartosik, 1962; Schwartz, 1964), by which time ovulation and vaginal cornification have occurred. A great deal of evidence suggests that these changes are due to a cyclic increase in ovarian secretion (see Schwartz, 1964). In a recent investigation of the timing of this ovarian secretion it was demonstrated that ovariectomy at 16.00 hours on the day before pro-oestrus, but not at 10.00 hours on the morning of pro-oestrus, blocked vaginal cornification; however, it was necessary to perform the ovariectomy at 10.00 hours on the day before

NEENA B. SCHWARTZ and WILLIAM L. TALLEY

Summary.

Daily measurements of pituitary lh content during pregnancy in the rat, by the ovarian ascorbic acid depletion method, reveal no evidence of the regular changes manifested during the non-pregnant oestrous cycle. Instead, lh content rises from typical oestrous values on Day 1 to levels characteristic of cyclic pro-oestrus on Day 8. Pituitary lh content then exceeds pro-oestrous levels throughout the remainder of pregnancy until Day 21. Ovarian weight first decreases up to Day 10, then rises until the end of pregnancy. Ovarian histology reveals the presence of follicles of all sizes on each day, accompanied by increasing size of the corpora lutea of pregnancy. The significance of these observations and previous data in the literature concerning the pituitaryovarian axis during pregnancy are discussed from the point of view of the question of persistence of some manifestations of the oestrous cycle during pregnancy.